Money saved (7)
'money saved' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nest egg that I've seen before include "Fund put by as a reserve" , "Money kept in reserve" , "Retirement savings" , "reserves" , "Money saved for a rainy day - by bird?" .)
(Another definition for nestegg that I've seen is " savings".)